The early days of ground-based gamma-ray astronomy in France

(1)

The early days of ground-based gamma-ray astronomy in France

Gerard Fontaine - Hillas symposium – Heidelberg – December 10-12 2018

(2)

Timeline from 1986 to 2004

86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04

THEMISTOCLE & ASGAT

CAT

CELESTE

Blois 1992

Palaiseau 1992 ICRC + Kruger 1997

(3)

It all started from controversial observations !

•  Claim from EAS experiments (Kiel & Haverah Park) of PeV photons with anomalous muon production

3

- 28 scintillation counters, 1m

²

each - 60,000 showers over 16,775 hours

- In the direction of Cygnus X-3 (3,838 hours):

excess of 16.6 showers (4.4 σ ) with an angular resolution of 1°

+ characteristic 4.8 h signal modulation - Very high flux:

Φ

_{> 2 PeV}

= (7.4 ± 3.2) x 10

^-14

photons cm

^-2

s

^-1

- Muon content similar to hadronic showers...

(4)

THEMISTOCLE Concept

•  Tracking High Energy Muons In Showers Triggered On

Cherenkov Light Emission - Use gamma beams from cosmic sources to reveal possible new mechanism of

photo production at high energies (Cl. Ghesquière).

- Cherenkov detection allows a much better angular resolution;

- Use the atmosphere as a GigaTon calorimeter =>

better energy measurement;

- Themis site available with 200 heliostats (Ph. Goret – ASGAT expt).

April 1986: Letter of intent;

Dec. 1986: Proposal

(5)

Energy Scale aimed at by T ^HEMISTOCLE

5

(6)

Descoping from T ^HEMISTOCLE -300 to T ^HEMISTOCLE -18

Pilot Cherenkov experiment with 18 telescopes approved by IN2P3 in March 1988

≈ Elliptical field 280 x 190 m

Proposal (300 telescopes)

(7)

T ^HEMISTOCLE -18 Detector

Construction from 1988 to 1990

7

=> 80 cm Ø mirror + fast XP2020 PMT + preamp

+ constant fraction discriminator + 0.1 ns TDC +

ADC for each channel

(8)

T ^HEMISTOCLE -18 first results (½)

•  Event reconstruction: in the TeV energy range, the light wave front is well described by a cone

• Ready for science observations in Summer 1990

Mean time residual:

0.33 ns Fitting a 6 parameter cone is highly non-linear. Convergence is slow due

to the singularity at the apex and many ridges in the parameter space!

Laser calibrated timing:

a ≈ 0.17 ns b≈ 2.7 ns

(9)

T ^HEMISTOCLE -18 first results ( ² / ₂ )

9

Signal extraction (based on Timing):

Distribution of reconstructed directions, u

_D

, u

_R

: Euler angles, local orthonormal

coordinate system in mrd.

Maximum likelihood method to test the existence of a source signal against the hypothesis of pure background (OFF data).

First season on Crab (67 h / winter 1990-91):

95 gamma detected at 3.5 σ.

1991 data on Her X-1 and Cyg X-3: NO signal With additional 95 h (1991-92) on Crab:

282±54 gamma at ≈ 6 σ, with an angular resolution of 2.2 mrad (0.13°).

( ICHEP Dallas Feb 1992,

XIII “Physics in Collision” Heidelberg 1993 )

Crab Nebula signal

(10)

T ^HEMISTOCLE energy mesurement & hadron rejection

•  Energy estimation and spectrum:

- The amplitudes of the signals from a shower are used in a global fit of the radial density distributions predicted by MC generation of gamma showers at different energies.

- In addition to the energy estimate, the quality of this fit provides a discrimination between gamma rays (good fit) and hadrons (bad fit).

- The energy scale is set by fitting the trigger rate (dominated by hadronic showers).

•  Publication in Astropart. Phys (1993) done with pure proton background has a biased energy scale !

The threshold is NOT 3 TeV as stated …

•  Roma ICRC paper (1995), with a more realistic background taking into account He and heavier ions.

A total of 388 h of Crab data => 407±68 gamma E > 2 TeV detected at 6.6 σ, with spectrum extending from 2 to 13 TeV.

Crab Nebula Integral Spectrum

22 publications in conference proceedings over 8 years, but only one in a referred journal!

(11)

T ^HEMISTOCLE lessons learned

•  On the technical side

- Wavefront sampling can be used for gamma-ray astronomy;

- Good reconstruction (direction and energy) requires a minimum of 12 samples for each shower;

- 18 telescopes was a bit too low in this respect and an array of 40 would have been much more efficient.

•  On the scientific side

- Confirmation that TeV gamma rays are emitted in the Universe;

- But their fluxes are much lower than anticipated and no emission is seen from Her X-1 and Cyg X-3;

- No hope for a discovery of a new mechanism of photo production at high energies, while at the same time the first results of the HERA collider were showing a conventional γ-p interaction up to 200 GeV CM energy.

•  Observations (almost) stopped in 1995

But a strong interest in gamma-ray sources had been triggered, and was going to continue …

11

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Gamma-ray astronomy status in 1992

•  Gamma-ray emission of the Crab Nebula

had already been discovered by the Whipple Obs. group (Ap J 1989) (Thanks to T. Weekes’ tenacity and to A.M. Hillas’ analysis method)

•  A.E Chudakov’s worries and satisfaction

at the Blois conference on “Particle Astrophysics” (1992):

(13)

Towards a Major Atmospheric Cherenkov Detector

13

And a wink to a concept of hybrid array with 54 Small sized,

16-30 Medium sized and 3-7 Large sized telescopes…

A Dream not yet true in 1992!

(G.F.)

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The CAT Project

•  After the 13

^th

ECRS conference in Geneva (1992) and the announcement of the HEGRA Cherenkov project :

the CAT imaging telescope was proposed in 1993 by B. Degrange et al for the Themis site:

- Imaging technique (following A.M. Hillas’ pioneering work) - Modest mirror size (16 m

²

)

- Very fast and fine grained camera (546 pixels 0.12° in Ø)

- Advanced analysis techniques based on image matching with a library of precomputed shapes.

•  First light in 1996: Despite a much smaller mirror size (16 m

²

) than that

of Whipple (75 m

²

), fine grained imaging made it possible for CAT to

reach a similar sensitivity and energy threshold (250 GeV)!

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CAT Telescope and Camera

15

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CAT Results and Timeline

•  Dec. 1992: Letter of Intent

•  July 1993: Proposal

•  1994: Approved by IN2P3

•  1996: First light

-  Nov. 1998: « The CAT imaging telescope… » NIM, vol. A 416, pp 278-292 -  Nov. 1998: « A new analysis method … » NIM, vol. A 416, pp 425-437 -  Oct. 1999: « VHE … Mkn 501 … in 1997 » A&A, vol. 350, pp 17-24 -  Aug. 2001: « Temporal … Mkn 421 … » A&A, vol. 374, pp 895-906

-  Aug. 2002: « Detect. … 1ES1426+428 …» A&A, vol. 391 (2002), pp L25-L28 -  2002: End of observations

-  2004: End of the experiment + 31 publications in conference proceedings over 9 years

incl. 1ES1959+650 (& 1ES1426+428) at Meudon 2003

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April 1997: An important Milestone

17

•  Simultaneous observation of the Mrk 501 flare by Whipple, H ^EGRA and CAT

Reported at the 25th ICRC in Durban and the Kruger Workshop (1997)

(18)

Revisiting wave-front sampling

•  Scientific need to bridge the energy gap [10 GeV – 250 GeV]

between space-borne and ground-based observations

•  Cherenkov technique below 100 GeV requires very large mirrors

•  Opportunity to use again the Themis solar plant array!

200 heliostats (54 m

²

each)

concentrating the sun’s power at the top of a

100 m height tower.

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The C ^ELESTE project

19

•  Wave-front sampling detector approved in 1996 as a multi-step project (6 -> 18 – > 40 mirrors ≈ 2000 m

²

) led by E. Paré

•  Required secondary optics on top of the tower

•  1 GHz FADCs

•  First light in 1998

(20)

C ^ELESTE results

Sensitivity down to 30 GeV

•  1998: « Prototype tests … » NIM, vol. A 412, pp 329-341

•  2002: « C

^ELESTE

: an atmospheric … » NIM, vol. A 490, pp 71-89

•  Feb. 2002: « Measurement … Crab … 60 GeV … » ApJ 566, pp 343-357

•  2006: « Mrk 421, Mrk 501 and 1ES 1426 … 100 GeV … _» A&A 459, 453-464

Crab nebula

gamma-ray

spectrum

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Moral of the story

21

The early days of ground-based gamma-ray astronomy in France

The early days of ground-based gamma-ray astronomy in France

Gerard Fontaine - Hillas symposium – Heidelberg – December 10-12 2018

Timeline from 1986 to 2004

86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04

THEMISTOCLE & ASGAT

CAT

CELESTE

It all started from controversial observations !

• Claim from EAS experiments (Kiel & Haverah Park) of PeV photons with anomalous muon production

- 28 scintillation counters, 1m

each - 60,000 showers over 16,775 hours

- In the direction of Cygnus X-3 (3,838 hours):

excess of 16.6 showers (4.4 σ ) with an angular resolution of 1°

+ characteristic 4.8 h signal modulation - Very high flux:

Φ

= (7.4 ± 3.2) x 10

photons cm

s

- Muon content similar to hadronic showers...

THEMISTOCLE Concept

• Tracking High Energy Muons In Showers Triggered On

Cherenkov Light Emission - Use gamma beams from cosmic sources to reveal possible new mechanism of

photo production at high energies (Cl. Ghesquière).

- Cherenkov detection allows a much better angular resolution;

- Use the atmosphere as a GigaTon calorimeter =>

better energy measurement;

- Themis site available with 200 heliostats (Ph. Goret – ASGAT expt).

April 1986: Letter of intent;

Dec. 1986: Proposal

Energy Scale aimed at by T HEMISTOCLE

Descoping from T HEMISTOCLE -300 to T HEMISTOCLE -18

Pilot Cherenkov experiment with 18 telescopes approved by IN2P3 in March 1988

≈ Elliptical field 280 x 190 m

Proposal (300 telescopes)

T HEMISTOCLE -18 Detector

Construction from 1988 to 1990

=> 80 cm Ø mirror + fast XP2020 PMT + preamp

+ constant fraction discriminator + 0.1 ns TDC +

ADC for each channel

T HEMISTOCLE -18 first results (½)

• Event reconstruction: in the TeV energy range, the light wave front is well described by a cone

• Ready for science observations in Summer 1990

Mean time residual:

0.33 ns Fitting a 6 parameter cone is highly non-linear. Convergence is slow due

to the singularity at the apex and many ridges in the parameter space!

Laser calibrated timing:

a ≈ 0.17 ns b≈ 2.7 ns

T HEMISTOCLE -18 first results ( 2 / 2 )

Signal extraction (based on Timing):

Distribution of reconstructed directions, u

, u

: Euler angles, local orthonormal

coordinate system in mrd.

Maximum likelihood method to test the existence of a source signal against the hypothesis of pure background (OFF data).

First season on Crab (67 h / winter 1990-91):

95 gamma detected at 3.5 σ.

1991 data on Her X-1 and Cyg X-3: NO signal With additional 95 h (1991-92) on Crab:

282±54 gamma at ≈ 6 σ, with an angular resolution of 2.2 mrad (0.13°).

( ICHEP Dallas Feb 1992,

XIII “Physics in Collision” Heidelberg 1993 )

Crab Nebula signal

T HEMISTOCLE energy mesurement & hadron rejection

• Energy estimation and spectrum:

- The amplitudes of the signals from a shower are used in a global fit of the radial density distributions predicted by MC generation of gamma showers at different energies.

- In addition to the energy estimate, the quality of this fit provides a discrimination between gamma rays (good fit) and hadrons (bad fit).

- The energy scale is set by fitting the trigger rate (dominated by hadronic showers).

• Publication in Astropart. Phys (1993) done with pure proton background has a biased energy scale !

The threshold is NOT 3 TeV as stated …

• Roma ICRC paper (1995), with a more realistic background taking into account He and heavier ions.

A total of 388 h of Crab data => 407±68 gamma E > 2 TeV detected at 6.6 σ, with spectrum extending from 2 to 13 TeV.

22 publications in conference proceedings over 8 years, but only one in a referred journal!

T HEMISTOCLE lessons learned

• On the technical side

- Wavefront sampling can be used for gamma-ray astronomy;

- Good reconstruction (direction and energy) requires a minimum of 12 samples for each shower;

- 18 telescopes was a bit too low in this respect and an array of 40 would have been much more efficient.

• On the scientific side

- Confirmation that TeV gamma rays are emitted in the Universe;

- But their fluxes are much lower than anticipated and no emission is seen from Her X-1 and Cyg X-3;

•  Claim from EAS experiments (Kiel & Haverah Park) of PeV photons with anomalous muon production

•  Tracking High Energy Muons In Showers Triggered On

Energy Scale aimed at by T ^HEMISTOCLE

Descoping from T ^HEMISTOCLE -300 to T ^HEMISTOCLE -18

T ^HEMISTOCLE -18 Detector

T ^HEMISTOCLE -18 first results (½)

•  Event reconstruction: in the TeV energy range, the light wave front is well described by a cone

T ^HEMISTOCLE -18 first results ( ² / ₂ )

T ^HEMISTOCLE energy mesurement & hadron rejection

•  Energy estimation and spectrum:

•  Publication in Astropart. Phys (1993) done with pure proton background has a biased energy scale !

•  Roma ICRC paper (1995), with a more realistic background taking into account He and heavier ions.

T ^HEMISTOCLE lessons learned

•  On the technical side

•  On the scientific side

•  Observations (almost) stopped in 1995

•  Gamma-ray emission of the Crab Nebula

•  A.E Chudakov’s worries and satisfaction

•  After the 13

•  First light in 1996: Despite a much smaller mirror size (16 m

•  Dec. 1992: Letter of Intent

•  July 1993: Proposal

•  1994: Approved by IN2P3

•  1996: First light

-  Aug. 2002: « Detect. … 1ES1426+428 …» A&A, vol. 391 (2002), pp L25-L28 -  2002: End of observations

-  2004: End of the experiment + 31 publications in conference proceedings over 9 years

•  Simultaneous observation of the Mrk 501 flare by Whipple, H ^EGRA and CAT

•  Scientific need to bridge the energy gap [10 GeV – 250 GeV]

•  Cherenkov technique below 100 GeV requires very large mirrors

•  Opportunity to use again the Themis solar plant array!

The C ^ELESTE project

•  Wave-front sampling detector approved in 1996 as a multi-step project (6 -> 18 – > 40 mirrors ≈ 2000 m

•  Required secondary optics on top of the tower

•  1 GHz FADCs

•  First light in 1998

C ^ELESTE results

•  1998: « Prototype tests … » NIM, vol. A 412, pp 329-341

•  2002: « C

•  Feb. 2002: « Measurement … Crab … 60 GeV … » ApJ 566, pp 343-357

•  2006: « Mrk 421, Mrk 501 and 1ES 1426 … 100 GeV … _» A&A 459, 453-464

•  Difficulties with the C ^ELESTE solar farm approach:

•  Go for imaging!